I personally only count for the uphill, many of my athletes use treadmills or stair masters, and I don’t cut their TSS for not going downhill. The biggest thing with all of these fudge factors they are made up. We have found after years of experience that this is roughly what they account for. I like keeping it simple and consistent. It is easiest to remember to add 10TSS for every 1,000ft, add 10TSS for 10% of body weight etc…It is easy to get into the weeds with these sorts of things.
With that being said, as long as you are personally consistent with them, you will be getting accurate data. I hope that helps!